YouTube has some limitations for different devices. For example, Android devices until recently were able to play videos at a maximum resolution of 1440p, even if the video was uploaded in 4K. This limit has been increased to 2160p (4K) resolution and HDR support.
Google has not officially announced this innovation, but an increasing number of users on the network indicates the update distribution. Apparently, the update comes from the server-side. For example, you can still play videos at a maximum of 1440p, but another user can have a resolution of 2160p.
4K resolution in the YouTube app is available even to users with HD displays in smartphones. This is a plus because videos with a resolution higher than the screen will look sharper on them and have a higher bit rate than videos with a native resolution.
Do you already have 4K support in the YouTube app?